Biological waste is produced e.g. in hospitals, agricultural or biological research and production facilities, plasma fractionation facilities, etc. Biological wastes produced in such facilities cannot be directly conducted to a sewer system, as these wastes often contain micro-organisms, such as bacteria, viruses and other microorganisms, which are hazardous to humans and animals. Prior to conducting to a sewer system, such biowaste must first be deactivated in a treatment plant designed for this purpose. For the treatment of biowaste, different treatment plants have been designed in which biowaste is sterilized prior to conducting to the sewer system. The sterilization of biowaste can be carried out chemically or by means of heat. The treatment plants can operate continuously or batchwise.
In an article by Carl J. Carlson in Pharmaceutical Engineering, May/June 2001, pages 70 to 82, facilities for the treatment of biowaste are described. The article deals with biowaste treatment facilities of different types as well as with dimensioning principles and problems relating thereto.
According to said article, a typical thermal continuous biowaste sterilisation apparatus comprises a separating unit for solid matter, a storage tank, a heating unit and a dwell circuit as well as a circulation circuit for circulating biowaste through said heating unit and said dwell circuit. According to the article, a typical continuous apparatus comprises the following stages: a heating stage, whereby biowaste is circulated in a heat exchanger and in a dwell circuit, until a temperature sufficient to kill the micro-organisms is reached. This is followed by an operating stage when the biowaste has reached the required temperature over the whole length of the heat exchanger. Thereby the treated biowaste is conducted through cooling equipment to a sewer system. If one or several sterilization parameters (temperature in the dwell circuit, pressure etc.) go outside the predetermined value, and the biowaste is therefore insufficiently sterilized, the process enters a hold state, where the biowaste is circulated through the heating unit and the dwell circuit until the parameter or parameters in question are again within the given limits. In case of an alarm, the apparatus enters the cooling mode, in which the operation of the heating unit is stopped, and the biowaste is recycled back to the pump feed line until the apparatus is again in working order. According to the article, provisions for the steam sterilization of the parts downstream from the storage tank should be provided, as well as provisions for preventing the transfer of the active biowaste to the cooling circuit. In addition, steam sterilization of the storage tank, the piping, venting filters, etc. should be provided in the apparatus.
Biowaste sterilization plants are usually provided with heat exchangers for cooling the effluent before it enters the sewer. The heat removed from the effluent stream is normally not utilized for feed preheating because of the safety issues involved with establishing a heat transfer connection between a sterilized stream and a biologically hazardous stream. A leak in a heat transfer loop may cause a serious risk of contamination.
The present invention provides heat recovery from the effluent stream without jeopardizing the integrity of the clean, or sterilized, stream and the equipment for handling it.